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Flea Beetles Attacking Brassica Plants in Virginia James A. C. Mason and Thomas P. Kuhar Department of Entomology, Virginia Tech

Classification: Order: Coleoptera, Family: Chrysomelidae, Subfamily: Galerucinae, Genus: Phyllotreta Identification: Small beetles ranging from 1.8-2.5mm in length. All flea beetles exhibit an enlarged hind femur (Fig. 1B) that enables their characteristic flea-like jump. The crucifer flea beetle (Phyllotreta cruciferae Goeze) has a metallic blue-black and oval shaped body covered in small holes (Fig. 1A). The striped flea beetle (Phyllotreta striolata Fabricius) has a mostly black body that appears slightly glossy, and typically features two irregular yellow stripes running down the length of the elytra, the hardened outer wings that cover the abdomen (Fig. 1B). The shape of the beetle is similar to that of the crucifer flea beetle and it also bears the same small holes on its exoskeleton. The striped flea beetle can often be confused with other similar beetles in the same genus.

Fig. 1. A) Crucifer Flea Beetle (Phyllotreta cruciferae), and the B) Striped Flea Beetle (Phyllotreta striolata). The arrow shows the enlarged hind femur.

Life History: The crucifer flea beetle has one to two generations per year in Virginia, although it can have more depending on climate and region. The beetle can complete its life cycle from egg to adult in roughly six or seven weeks (Capinera 2001). The striped flea beetle also has one to two generations per year in or more, especially in warmer areas such as the southern United States. This is due to the relatively short time from egg to adult taking only four to five weeks to complete its life cycle (Capinera 2001). Both beetles lay their eggs at the base of the host plant. The eggs are laid in the spring as soon as the adults emerge from overwintering and are laid throughout the season. For crucifer flea beetles the eggs can take between 11 and 13 days to hatch, the striped flea beetle hatches in as little as five days (Capinera 2001). The larvae of both species burrow into the ground and feed on the root hairs of the host plant they are laid under. The crucifer flea beetle spends up to 30 days as a larva, and the striped flea beetle spends less time in this stage (Capinera 2001). The pupal stage lasts about 6 to 9 days in both species. The adults emerge

from the soil after the pupal stage and are able to disperse or feed on nearby host plants. Both flea beetles overwinter as adults in leaf litter, soil, or debris in and around the area of their host plants. During the growing season both beetles are often found feeding together on the same plants. Hosts: Both the crucifer flea beetle and the striped flea beetle are specialized to feed on brassica plants. They feed on cabbage as well as broccoli, Brussels sprouts, cauliflower, Chinese cabbage, collards, horseradish, kale, kohlrabi, mustard, radish, rutabaga, and turnip. Distribution: The crucifer flea beetle ranges from Canada to northern United States, it is also found along much of the east coast of the United States (Capinera 2001). The information on which states it is found in is not current and it could have spread across much of North America. The striped flea beetle is often found in many of the same areas as the crucifer flea beetle, however is has a larger range inhabiting much of the United States and Canada (Capinera 2001). These beetles can be found all over Virginia but typically do not thrive in sandy soil. Description of Damage: These two flea beetle species injure cabbage leaves by chewing small holes in them, they do not eat all the way through the leaf, but can still cause significant damage (Fig. 2). Depending on the host plant, flea beetles can cause indirect or direct damage to the plant. Direct damage occurs when the leaf is the marketable part of the plant, such as collards or kale, these leaves are left riddled with holes, become brittle, and begin to fall apart. Indirect damage is caused to brassicas where the flower heads are the marketable parts of the plant, damaged leaves reduce the photosynthetic capability of the plant, which can drastically reduce yield at harvest (Fig. 3).

Fig. 2. Examples of flea beetle damage to cabbage. A) A severely defoliated cabbage plant, and B) flea

beetles attacking the underside of a cabbage leaf. Control: Chemical control: The primary control method for flea beetles involves using insecticides either preemptively or after a major flea beetle outbreak has occurred. Systemic neonicotinoids particularly dinotefuran and imidicloprid can be applied through the transplant water at the time of planting (Mason

and Kuhar 2015). These insecticides can protect the plant up to 5 weeks during the early vulnerable stages when flea beetles cause most of their damage. Also, pyrethroid foliar sprays can significantly reduce flea beetle populations to levels that cause little to no damage (Mason and Kuhar 2015). They should be applied only when beetles are present in compliance with the insecticide label. Non-chemical control: When possible plant date can be adjusted to avoid peak beetle populations, beetles emerge and begin feeding toward the end of May and early June, if the plants are transplanted before this emergence, they can outgrow much of the damaging period that comes toward the end of June. Since flea beetles overwinter in surrounding leaf litter and debris, much of this can be removed or cleaned up in the fall to provide less habitat for overwintering adults and reduce the number of emerging flea beetles in the spring. Row covers can be used to protect smaller plants, but should probably not be left on the crop for long in order to not impact plant growth. There is no effective biological control for flea beetles.

Fig. 3. Cabbage heads at harvest time ranging from plants with no flea beetle feeding injury (left) to plants

with major feeding injury (right).

Literature Citations Capinera, J. (2001). Handbook of vegetable pests. Elsevier. Mason, J. A., & Kuhar, T. P. (2016). Evaluation of Insecticides for the Control of Flea Beetles in Cabbage, 2015. Arthropod Management Tests, 41(1), tsw013.